1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image-forming apparatus using the optical scanning apparatus.
2. Description of the Related Art
Various optical scanning apparatus for use in image-forming apparatuses (e.g., laser beam printers, digital copy machines, and multifunction printers that perform electrophotography processes) have been suggested (see Japanese Patent Laid-Open No. 2003-156704).
In this type of optical scanning apparatus, a light beam emitted from a light source unit including a semiconductor laser is collimated by a collimating lens and is guided to a deflecting-reflecting surface (deflecting surface) of a light deflector including a rotating polygon mirror. The light beam deflected by the light deflector is caused to form a spot image on a surface by an imaging optical system (fθ lens system), and this surface is scanned with the light beam at a constant speed. This type of optical scanning apparatus includes a so-called surface-tilt-correction optical system in which the substantially collimated light beam output from the collimating lens is collected at or near the deflecting-reflecting surface in a sub-scanning direction (sub-scanning cross section) perpendicular to the deflecting direction (main-scanning direction) by a cylindrical lens and is then caused to form the spot image on a surface to be scanned by the imaging optical system.
Recently, demand for high-speed and high-definition printing performance of image-forming apparatuses (e.g., laser beam printers, digital copy machines, and multifunction printers) has increased. To achieve either high-speed printing or high-definition printing, it can be necessary in some circumstances to increase the number of times the surface is scanned per unit time. Accordingly, the number of surfaces and the rotating speed of the rotating polygon mirror have been increased.
However, in this case, the size of the rotating polygon mirror and load placed on a drive motor are increased. Therefore, new problems arise that the temperature and noise are increased and the overall size cannot be reduced.
Accordingly, in order to reduce the load placed on the light deflector, various types of multi-beam scanning methods have been suggested in which the number of light-emitting portions included in a semiconductor laser that can function as the light source unit is increased and a plurality of light beams are contemporaneously deflected and caused to scan a surface to be scanned.
There are two major types of light sources used in the multi-beam scanning methods:                A first type in which a plurality of light-source elements which each emit a single laser beam can be arranged and a plurality of light beams can be obtained using optical-path-combining units (e.g., polarizing beam splitters and half mirrors); and        A second type called a monolithic multi-beam type in which a plurality of light-emitting portions can be arranged on a single light-source element.        
Although light sources of the first type can be easily manufactured using simple (inexpensive) single laser emitting elements, there is a problem that the overall structure is complex and large because the beam-combining units are necessary. In comparison, in the monolithic multi-beam type, no beam-combining unit is necessary and accordingly the structure of the optical scanning apparatus can be made simpler and smaller.
There are two major types of monolithic multi-beam light-source elements: horizontal emission type and vertical emission type. Each type of light-source element is manufactured by a semiconductor process and has a layered structure formed on a wafer substrate. The beam is emitted horizontally from the layered structure in the horizontal emission type and vertically in the vertical emission type.
In general, semiconductor lasers of the horizontal emission type are mainly used because they can be easily manufactured. In multi-beam light sources of the horizontal emission type, beams can be arranged one-dimensionally. The horizontal emission type is also called an edge emitter type.
In the vertical emission type, light-emitting portions can be arranged two-dimensionally on the substrate surface because the light beams are emitted vertically with respect to the substrate surface. Accordingly, the laser sources of this type are called Vertical Cavity Surface Emitting Lasers. The Vertical Cavity Surface Emitting Lasers are advantageous in that the number of light-emitting portions can be easily increased by arranging them two-dimensionally, and have recently been attracting considerable attention.
On the other hand, optical elements included in imaging lenses of the optical scanning apparatus are generally formed by molding using a mold. Molding is advantageous in that lenses having complex shapes can be easily manufactured with high reliability once the mold is obtained. Accordingly, optical elements having aspherical surfaces are often manufactured by molding so that the optical performance can be increased and the number of lenses can be reduced. In particular, various lens structures having surfaces aspherical in the main-scanning direction have been suggested to reduce the comma aberration and improve the fθ characteristics.
In addition, various kinds of optical scanning apparatuses including lens surfaces aspherical in the sub-scanning direction have also been suggested (see Japanese Patent Laid-Open Nos. 2001-021824, 2-157809, 9-90254, 2000-121977, and 2004-70108).
The above-mentioned optical scanning apparatuses provide two major effects:                Wave aberration (spherical aberration) in the sub-scanning direction is reduced (Japanese Patent Laid-Open Nos. 2001-021824, 2-157809, and 9-90254); and        Scan-line curvature is reduced (Japanese Patent Laid-Open Nos. 2000-121977 and 2004-70108).        
The structures according to Japanese Patent Laid-Open Nos. 2001-021824, 2-157809, and 9-90254 compensate for a displacement between a paraxial image plane and a best-spot image plane caused by the influence of the spherical aberration generated due to an increase in the width of the light beam in the sub-scanning direction.
In the structures according to Japanese Patent Laid-Open Nos. 2000-121977 and 2004-70108, the light beam incident at an angle passes through an imaging lens surface at a position separated from the optical axis in the sub-scanning cross section. Accordingly, the irradiation height of the image point on the surface to be scanned is largely shifted from the optical axis due to the spherical aberration of the imaging lens, which generates the scan-line curvature. The above-mentioned structures are provided to reduce this scan-line curvature.
The above-described Vertical Cavity Surface Emitting Laser that emits an increased number of beams from two-dimensionally arranged light-emitting portions can have a certain field angle to reduce jitter in the main scanning direction.
The jitter in the main-scanning direction will be explained below. Since the light-emitting portions included in the laser chip are separated from each other and gaps with a certain width are provided between the spots in the main-scanning direction, two light beams propagate at an angle with respect to each other in the main-scanning direction. Accordingly, the polygon mirror is at different rotational positions when the two light beams are incident on (scan) the same point on the photosensitive drum in the main-scanning direction, which means that the two light beams can be incident on that point at different times. Therefore, the positions (distances from the optical axis) at which the two light beams pass through the imaging lens (fθ lens) also differ from each other in the main-scanning direction, and sufficient effects cannot be obtained due to differences between the positions at which the light beams pass through the imaging lens in the main-scanning direction. In other words, since the principal rays of the light beams are incident on the polygon mirror at different positions, the light beams that travel toward the same image height in the main-scanning direction pass through the imaging lens at different positions. This causes the jitter in the main-scanning direction.
The jitter in the main-scanning direction can be reduced by arranging the laser source such that the field angle in the main-scanning direction is reduced, that is, such that the field angle in the sub-scanning direction is increased.
However, when the field angle in the sub-scanning direction is increased, the following problems occur:                A field curvature between the beams occurs in the sub-scanning cross section; and        The gaps between the beams become uneven due to a distortion (DIST) in the sub-scanning cross section.        
For example, FIGS. 21 and 22 show aberrations obtained when a collimating lens with a focal length (F) of 16.3 and a cylindrical lens with a focal length (F) of 36.0 in the sub-scanning direction are included in the incident optical system according to Japanese Patent Laid-Open No. 2003-156704 and a Vertical Cavity Surface Emitting Laser having a field angle in the sub-scanning direction is used as a laser source.
FIG. 21 illustrates a graph of the paraxial image plane in the sub-scanning direction, where the vertical axis illustrates the paraxial image plane in the sub-scanning direction (sub-scan image plane) and the horizontal axis illustrates the image height (scan image height) on a surface to be scanned in the main-scanning direction. The graph illustrates the case in which the light-emitting portions of the laser source can be arranged such that the field angle is varied with 0.02 mm pitch in the range of Z=0.000 mm to 0.100 mm in terms of the distance from the optical axis of the collimating lens in the sub-scanning direction.
As is clear from FIG. 21, as the field angle in the sub-scanning direction (sub-scan field angle) of the light-emitting portions is increased, the sub-scan image plane is shifted in the negative direction and accordingly a field curvature occurs.
The sub-scan image plane is particularly largely shifted in a region where the scan image height is near the axis. The sub-scan image plane is curved with respect to the sub-scan field angle (Z=0.000 mm to 0.100 mm for the field angle of the laser source).
Although the shift of the sub-scan image plane is small if the sub-scan field angle is small (around Z=0.02 mm for the field angle of the laser source), it cannot be ignored when the Vertical Cavity Surface Emitting Laser is used and the sub-scan field angle is increased.
FIG. 22 illustrates a graph of the irradiation height of the image point on a surface to be scanned in the sub-scanning direction, where the horizontal axis illustrates the image height on the surface to be scanned in the main-scanning direction (scan image height) and the vertical axis illustrates the irradiation height of the image point in the sub-scanning direction. The graph illustrates the case in which the light-emitting portions of the laser source can be arranged such that the field angle is varied with 0.02 mm pitch in the range of Z=0.000 mm to 0.100 mm in terms of the distance from the optical axis of the collimating lens in the sub-scanning direction.
As is clear from FIG. 22, when the sub-scan field angle of the light-emitting portions is large, the irradiation height of the image point in the sub-scanning direction is shifted in the positive direction as the image height in the main scanning direction is increased. Accordingly, a scan-line curvature occurs.
This means that the gap between the beams in the sub-scanning direction (sub-scan pitch) varies depending on the main-scan image height.
The amount of variation is particularly large in regions where the scan image height is large, and distortion (DIST) in the sub-scanning direction occurs in these regions.
Although the variation in the sub-scan pitch between the beams is small if the sub-scan field angle is small (around Z=0.02 mm for the field angle of the laser source), it cannot be ignored when the Vertical Cavity Surface Emitting Laser is used and the sub-scan field angle is increased.
Japanese Patent Laid-Open No. 2001-021824 discusses an aberration correction structure for a light source emitting a plurality of beams arranged such that the light source has a field angle in the sub-scanning direction. However, in this structure, the field angle in the sub-scanning direction is assumed to be around ±0.021 mm or less, which corresponds to the cases where the field angle is very small in the graphs shown in FIGS. 21 and 22.
In addition, the specification of Japanese Patent Laid-Open No. 2001-021824 discusses no concept for compensating for the differences in aberrations between the beams.
In addition, there is another problem in that the influence of wave aberration is increased when the beam diameter in the sub-scanning direction is increased to reduce the spot size.
To correct this, in the structure according to Japanese Patent Laid-Open No. 2001-021824, a surface where the beam diameter in the sub-scanning direction is at a maximum is designed to be aspherical. However, this is not sufficient for a light source arranged to have a field angle in the sub-scanning direction.
This is because a light beam with a field angle in the sub-scanning direction can cause a coma aberration when the light beam passes through an optical surface at a position separated from the optical axis and the coma aberration is increased as the light beam width is increased.
Therefore, the comma aberrations caused by light beams that pass through a lens surface at different positions cannot be sufficiently reduced by the structure according to Japanese Patent Laid-Open No. 2001-021824.